This application claims the priority of Germany application 196 39 052.4-24 filed in Germany on Sep. 24, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention is based on cast thin-walled structural elements and on a method for manufacturing thin-walled structural elements by casting from alloy for bodies utilizing casting techniques as known for example from a paper by W. Bovensmann, "Modern Diecasting Machinery for Precise High-Volume Diecastings" in the journal Gie.beta.erei-Praxis, No. 7, 1966, pages 115 to 119.
Although only diecasting is mentioned and described in detail in this literature reference from 1966, other casting methods of different types have recently been considered as well. Mention should be made in this connection of diecasting using an evacuated diecasting mold and thixocasting as well as thixoforming. In general, this refers to a creative forming method in which a quantity of metal adjusted for the size of the workpiece is placed in a shaping tool that can be opened and closed and is made of metal, and the mold is then filled with the flowable alloy and shaped under the influence of high pressure, after which the workpiece is left to set under the influence of this pressure until the workpiece can be handled, whereupon it is removed from the mold. All of the shaping methods that can be subsumed under this definition are covered here by the term "diecasting." In this connection, mention can be made for example of the special edition of the journals ATZ and MTZ in 1996 on the topic "Alloys in Automobile Construction" with papers by B. Wendiger and A. Mundl: "Thixocasting --A Shaping Technique with a Future," or G. Springe: "The Future of the Use of Aluminum in Automobile Construction." With the proper choice and pretreatment of the pre-product with globulitic structure and exact-temperature heating of the cylinder block to be used, the casting material behaves thixotropically, in other words it can be handled carefully as a solid but begins to flow under the influence of force. In thixocasting, the partially liquefied block is placed in a piston chamber connected with the casting mold by a feed channel, from which chamber the trixotropic metal is injected into the mold under high pressure, as in diecasting, by means of a piston. In thixoforming on the other hand, the partially liquefied block is placed directly in the mold which is still open and, as in drop forging, the impressions in the die are filled with partially liquefied metal by powerful and rapid closing of the halves of the mold.
There are natural limits in this method as to the size of the castings that can be produced. Firstly, because of the high pressure required, up to 1600 bars, and because of the projected area of the surfaces of the casting mold that are wetted by metal, very considerable forces act on the casting machinery and the mold which, because of the material elasticity of the steel according to Hook's law, lead to minor yet unavoidable deformations. With increasing size of the casting mold, despite increasing reinforcement of the machine and the mold, these deformations become so great that the mold can no longer be closed cleanly and free of flash. Another natural limit to the size of the castings, especially thin-walled castings, is the maximum possible flow distance of the metal, in other words, the maximum distance that the material can flow during filling of the mold before it hardens. Despite high flow rates, at least with narrow mold cross sections, due to contact with the mold walls, cooling occurs that increases with the length of the flow path. It should be considered in this connection that at high metal flow rates, the melt does not flow in the form of a cohesive stream of liquid but is atomized into many individual droplets that are then pressed together by the high mold-filling pressure. If the melt particles have flow paths that are too long, they cool off too much and lose their ability to adhere when pressed together, so that the casting becomes porous and brittle. In casting methods of the type described here, especially diecasting, thixocasting, and thixoforming, for the reasons given above, there are limits to the size of the castings that reflect the method used but cannot be expressed in units of length because they also depend on other factors, especially the type of casting, its shape, the wall thickness, and the accuracy requirements.
The goal of the invention is to improve the structural component produced according to the species or the manufacturing method in such a way that thin-walled cast structural elements can also be manufactured using the size limits associated with the method.
This goal is achieved on the basis of the species according to the invention by providing an automobile structural element which consists of a plurality of similarly cast casting slices which each extend over a partial area of the structural element, which casting slices are connected shapewise and materialwise along their facing seams. This goal is further achieved by using a method of casting thinwalled automobile body structural elements in which a quantity of metal adjusted for the size of the casting is placed centrally into a casting mold made of metal, the flowable alloy is shaped inside the mold under the action of high pressure, and the casting is then allowed to harden under this pressure until the workpiece can be handled and removed from the mold, wherein the castings are each cast in the form of several casting slices, with the casting slices of a casting being cast staggered timewise in succession, using the same method and an at least approximately identical alloy but using different casting molds or different sections of a casting mold, said slices being cast together at their opposing seams along a predefined shapewise contour.
According to the method, the castings are cast in several casting slices manufactured in succession with a time delay, with these slices being cast together at their opposite seams along a specified contour with an integral shape.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.